1. Field
The present disclosure relates to a nitride semiconductor device and a quantum cascade laser using the nitride semiconductor device.
2. Description of the Related Art
Light emitting diodes (LEDs) and laser diodes (LDs) serving as semiconductor light emitting devices that use a nitride semiconductor are used for illuminating devices, display devices, optical recording devices, and the like. Furthermore, electronic devices that use a nitride semiconductor are used for high-frequency devices and power devices. Quantum cascade lasers (QCLs) are promising for next-generation terahertz light sources.
In mass-produced nitride semiconductor devices, layers are formed on a c-plane ((0001) plane) serving as a principal surface. However, since the c-plane is obtained by alternately stacking Ga layers and N layers, polarization occurs in a growth direction. If polarization discontinuity arises because of heterojunction, charges are induced at heterointerfaces. The generation of an internal electric field due to the charges causes band bending.
The band bending is known to adversely affect device characteristics. For example, the band bending causes separation of electrons and holes in light emitting devices and exhibits normally-on characteristics in electronic devices. In devices, such as quantum cascade lasers, in which a plurality of subbands are coupled with each other, band bending considerably complicates the design of subbands or increases the susceptibility to production error.
An m-plane ({1-100} plane) known as a nonpolar plane is a-plane perpendicular to the c-plane. Ga and N are present on the principal surface with the same number, and therefore polarization is not generated in a direction of the normal to the m-plane. As a result, the above-described band bending does not occur. Accordingly, the m-plane is believed to be suitable for light emitting devices and electronic devices, and thus has been mainly used for light emitting devices including an active layer made of InGaN (e.g., refer to Okamoto Kuniyoshi, Ohta Hiroaki, Nakagawa Daisuke, Sonobe Masayuki, Ichikawa Jun, Takasu Hidemi, “Dislocation-Free m-Plane InGaN/GaN Light-Emitting Diodes on m-Plane GaN Single Crystals”, Japanese Journal of Applied Physics, 45, L1197 (Non-Patent Document 1)).
An InGaN quantum well structure is suitable for visible light devices because the band gap corresponds to a wavelength longer than the wavelength of blue light. A quantum well structure in which a GaN quantum well layer is sandwiched by AlGaN, which has a larger band gap than GaN, has received attention for use in ultraviolet light devices and intersubband transition devices in an infrared to terahertz range. AlGaN has a smaller lattice constant than GaN. In the case where the AlGaN is subjected to coherent growth on a GaN substrate, when stress in a film reaches its limit as a result of exceeding the critical thickness of the film, defects are introduced, which causes lattice relaxation of the AlGaN. If cracks are formed in a layer (e.g., an active layer) that exhibits device characteristics as a result of lattice relaxation, the device is electrically divided. This causes nonuniform light emission in optical devices and conduction failure in electronic devices. The axes perpendicular to each other in an in-plane direction of a principal surface of an m-plane are a c-axis (<0001>) and an a-axis (<11-20>). Different lattice constants are present in the in-plane direction of the principal surface, and thus AlGaN has strain anisotropy in the in-plane direction. A c-plane substrate used for known LEDs and the like does not include a c-axis in its principal surface and includes only an a-axis. The following has been found from the experiment conducted by the present inventors. When a thick AlGaN layer (about 1 μm) is grown on GaN, hexagonal cracks are formed in the AlGaN on a c-plane substrate, but cracks extending parallel to the a-axis, that is, linear cracks extending along the c-plane are often formed in the AlGaN on the m-plane substrate. In other words, the crack formation mechanism differs between the m-plane substrate and the c-plane substrate.
Japanese Unexamined Patent Application Publication No. 2008-277539 (Patent Document 1) discloses a method for suppressing the formation of cracks in AlGaN on an m-plane GaN substrate. In the method, an AlGaN layer having an Al content of 0.05 or more and a thickness of 500 nm or more is formed on the m-plane GaN substrate. According to the inventors in Patent Document 1, when an AlGaN layer having an Al content of 0.05 or more and a thickness of 500 nm or more is grown, microscopic lattice relaxation on a c-plane, which is a slip plane, occurs in the AlGaN layer. This allows lattice relaxation of the AlGaN without forming cracks. Furthermore, when the AlGaN layer has an Al content of 0.1 to 1.0 and a thickness of 2 μm or more, large strain is contained, but the AlGaN layer is not relaxed as a result of the formation of cracks.
However, it has been found from the experiment conducted by the present inventors that when an AlGaN layer having an Al content of 0.2 and a thickness of 1.2 μm, which is an example of the above configuration, is grown on an m-plane GaN substrate, lattice relaxation due to cracks occurs under some growth conditions, which makes it difficult to produce a device.